The present invention concerns a catalyst for the treatment of exhaust gases from an internal combustion engine, more particularly a compression ignition engine.
Such exhaust gases contain carbon monoxide, unburned hydrocarbons, nitrogen oxides and, in the case of compression ignition engines, particles, all of which are considered to be environmental pollutants.
Particles present in exhaust gases have a carbon skeleton (also known as soot) on which other compounds are adsorbed: high molecular weight hydrocarbons which constitute the so-called soluble fraction, sulfates, sulfuric acid, water vapor and various metallic compounds.
Pollutant emissions, more particularly particle emissions, depend greatly on the type of engine (spark ignition, compression ignition, direct injection, pre-chamber . . . ) and on its operating conditions (in particular the exhaust temperature).
In the case of compression ignition engines for light vehicles, exhaust gas temperatures during testing using a standard European cycle are relatively low at between 100.degree. C. and 300.degree. C., but for heavy vehicles (direct injection Diesel engines), they reach higher temperatures (up to 600.degree. C.) at certain points in standard European cycle R 49. In addition, even in light vehicle engines operating under real conditions, exhaust temperatures can reach 600.degree. C. to 700.degree. C.
To limit these pollutant emissions, different means have been developed and installed in the exhaust system: catalysts which promote oxidation of carbon monoxide, unburned hydrocarbons and particles and, in certain cases, reduce the nitrogen oxides; and particle filters especially for use in compression ignition engines.
These catalysts are well known in the art and are generally formed from a monolithic ceramic or metal support on which a fine layer of one or more refractory oxides is deposited, generally alumina with a surface area and porosity considerably greater than in the monolithic substrate. The active metallic phase, composed essentially of platinum group metals (Pt, Rh, Pd, Os, Ru, Ir), is dispersed in this oxide.
These catalysts have to operate under severe conditions and they deactivate with time. A number of patents claim to limit this loss of activity, for example by adding other oxides such as rare earth oxides, alkali metal or alkaline-earth metal oxides to the catalyst.
In the case of spark ignition engines, European patent EP-B-0 027 069 describes the addition of cerium and iron to a catalyst constituted by platinum group metals deposited on an inorganic refractory oxide which increases thermal stability and resistance to ageing.
In the case of compression ignition engines, so-called oxidation catalysts reduce carbon monoxide, hydrocarbon and particle emissions. The catalyst types described in European applications EP-A-0 432 534, EP-A-0 462 593, EP-A-0 315 896 and EP-A-0 174 495, for example, include platinum group metals, more particularly platinum itself, deposited on an inorganic refractory oxide such as alumina, silica, aluminosilicates or titanium oxide. Other oxides can be added to promote certain reactions and increase thermal stability.
To reduce particle emission, filters can also be installed in the exhaust system. The use of catalytic filters means that regeneration can be carried out in situ, i.e., at lower temperatures than those required for combustion of soot in the absence of the catalytic phase. U.S. Pat. Nos. 4,849,339, 4,510,265 and 5,100,632 illustrate the state of the art as regards. As with oxidation catalysts, these devices generally comprise a refractory oxide layer on which one or more platinum group metals is deposited.
The catalysts described in the prior art, which reduce pollution from an internal combustion engine, generally contain concentrations of platinum and/or a platinum group metal of between 0.2 and 10 g of metals per liter of catalyst.
International Application WO 93/10886 describes oxidation catalysts used to treat exhaust gases, in particular from Diesel engines, comprising a support which is coated with a porous layer of cerium and alumina, the catalytic element which is deposited being platinum, in an amount of 0.1 to 5 g/cubic foot (about 3.53 to about 176.5 g/cm.sup.3).
One of the objects of the invention is to improve the properties of these catalysts.
The high catalytic activity of noble metals, in particular platinum, in oxidation reactions means that carbon monoxide and hydrocarbon emissions can be considerably reduced, but has the drawback of flavoring oxidation of the sulfur dioxide present in the exhaust gases, particularly in the case of compression ignition engines. The sulfur trioxide SO.sub.3 thus formed can either combine with water molecules to produce sulfuric acid which can condense on the particles or on the particle filter, or it can react with the catalyst support (alumina, titanium oxide, etc . . . ) to form sulfates which will decompose at high temperature and increase the particle emissions. Thus the consequence of these phenomena is an increase in the mass of particles emitted by the engine.
Catalytic formulations for depolluting internal combustion engines, in particular compression ignition engines, must also:
permit oxidation of the carbon monoxide, unburned hydrocarbons and particles under the engine operating conditions (i.e., at the lowest possible temperatures); PA1 prevent sulfate and sulfite formation due to oxidation of sulfur dioxide; and PA1 be thermally stable and resist ageing. PA1 0.3% to 4.4% of cerium; PA1 0.1% to 3.5% of iron; and PA1 0.0003% to 0.04% of platinum; and PA1 the complement to 100% being constituted by at least one inorganic refractory oxide. PA1 0.3% to 4.4% of cerium; PA1 0.5% to 2% of iron; and PA1 0.0035% to 0.03% of platinum; and PA1 the complement to 100% being constituted by at least one inorganic refractory oxide. PA1 oxidation catalysis for the treatment of exhaust gases from compression ignition engines; and PA1 more particularly, oxidation catalysis for the treatment of exhaust gases from commercial vehicles with a total unladen weight of 3.5 metric tons or more. The following examples illustrate the invention.